Commonly, in the process of manufacturing fermented products such as beer, yeast recovered from a fermentation tank is stored in the stirred tank, and then returned as seed yeast to the fermentation tank for reuse.
Yeast stored in the stirred tank settles down on a bottom of the stirred tank as the time lapses, resulting in non-uniform yeast concentration in the stirred tank or non-uniform temperature when cooled. Hence, there arises a necessity to stir yeast slurry for overcoming such non-uniformity.
However, likewise butter, soap, etc., the yeast slurry is a non-Newtonian fluid. It is known that in such non-Newtonian fluid, stirring effect in proportion to the increase of the stirring force cannot be necessarily produced even with enhanced force unlike Newton fluid, in which the stirring effect is enhanced in proportion to the stirring force.
On the other hand, there also exists a necessity to protect yeast from damages due to stirring, along with the necessity to perform stirring for uniform yeast concentration and uniform temperature of yeast slurry.
In such a conventional process of manufacturing beer, an inclined paddle impeller, propeller or the like has been mainly used as a stirring impeller provided within the stirred tank for storing yeast slurry to be supplied to the fermentation tank.
However, in case that yeast slurry which is a non-Newtonian fluid is stirred by using such a conventional stirring impeller, there arises a problem that uniformly mixing of the entire yeast slurry is unlikely to be achieved through low-speed stirring.
On the other hand, there arises another problem in high-speed and strong stirring which is performed for improved uniformity in the yeast concentration and temperature of the yeast slurry, that yeast may be damaged and destroyed, and hence its biological activity may be lowered.